The invention relates to a process for converting premilled quinacridone to pigmentary form, and, more particularly, to a process involving contacting premilled quinacridone with an aqueous alkaline medium in the absence of organic liquids.
It is well known in the art that quinacridones direct from synthesis, known as crude quinacridones, are unsuitable for use as pigments and must be further processed to develop appropriate pigmentary properties, e.g., particle size, strength, phase, etc. The most commonly used processes for converting crude quinacridone to pigmentary form involve milling the crude quinacridone with large quantities of inorganic salt, then acid extracting the resulting quinacridone pigment. While the salt-grinding process can produce satisfactory pigment, the total volume of the grinding elements and salt is so large that only a relatively small amount of pigment can be milled for each batch. Furthermore, disposal of the large amounts of salt solution which results from the extraction step creates potential pollution problems and adds considerably to the manufacturing costs.
Attempts to eliminate the use of salt in premilling crude quinacridone, as described, for example, in U.S. Pat. No. 2,857,400 and U.S. Pat. No. 3,017,414, involve milling crude quinacridone without salt and contacting the milled quinacridone with an organic liquid or an acidic aqueous slurry of organic liquid. The organic liquid is removed from the slurry and the resulting quinacridone pigment is isolated in the conventional manner. The amounts of organic liquid utilized in these processes are generally at least 30% by weight, based on the weight of the quinacridone being processed and, like the salt, can create expensive waste disposal problems.
Another method for preparing pigmentary quinacridone from crude quinacridone, as described in U.S. Pat. No. 3,287,147, involves heating a neutral aqueous paste of quinacridones under pressure at between 150.degree. C. and 300.degree. C. The quinacridones so treated can result from salt milling crude quinacridones or from acid pasting crude quinacridones. In either case special equipment is necessary to withstand the high temperatures and pressures required.
More recently, another method for preparing pigmentary quinacridone from crude quinacridone, as described in U.S. Pat. No. 4,094,699, involved contacting premilled crude quinacridone with an aqueous alkaline medium in the presence of at least one surfactant. While this process is generally satisfactory, some residual surfactant is retained on the pigment which can have an adverse effect on some end use systems. For instance they can cause undesirably high viscosity in some automotive paint systems and reduce the stability of vinyl plastics.